Several processes have been devised to produce articles of fused silica-containing materials utilizing the basic flame hydrolysis/oxidation reactions disclosed in U.S. Pat. No. 2,272,342. Two of those methods which have seen extensive commercial use have been denoted the hot forming and soot preform processes, respectively.
The hot forming process has been employed in the fabrication of very large masses of fused silica, e.g., mirrors for reflecting telescopes. In conducting that process, the silica soot is deposited over a large area rotating bait or mandrel and consolidated directly into non-porous glass utilizing multiple burners. It will be appreciated that considerable flame heating is required to offset the heat loss in such a large furnace.
Two major thermal fluctuations or gradients appear when doing this with multiple burners, and each adversely affects optical quality by causing refractive index alterations because of non-annealable glass structure inhomogeneities. One of those fluctuations results from the boule surface passing in and out of the deposition flames giving rise to hot spots. This phenomenon promotes a laminar structure or striae in the boule which has the concomitant effect of restricting the use of such glass articles to single-direction optics applications. The second fluctuation or thermal gradient, running from center-to-edge of the boule, is caused by furnace geometry. This non-uniformity has been well-nigh impossible to eliminate by burner placement, particularly adjacent to the boule center, and leads to non-annealable stress birefringence rings (often called Schlieren rings), thereby rendering the glass unacceptable for high grade optics.
Another major problem seemingly inherent in the use of the hot forming process for the production of large masses of fused silica is the formation of inclusions (platelets, seeds, blisters, etc.) which generally result from refractory dust, grit, etc. falling from the furnace environment onto the boule during manufacture. Such contamination appears to be an intrinsic byproduct of the hot forming process when refractory furnaces with large diameter crowns are involved.
Yet another problem encountered in the hot forming process is the growth, during the deposition step, of nodules (called "elks teeth") at the boule perimeter. The nodule formation occurs in any area of deposition where the temperature of glass deposition is too low for adequate glass flow, which is notably at the boule edges. This situation cannot readily be remedied without disrupting other critical parts of the deposition process. Thus, e.g., the whole furnace would be operating at too high a temperature.
In the soot preform process, a porous soot body (preform) is first prepared by flame hydrolysis on a bait or mandrel. The preform is then zone sintered in a furnace utilizing a helium atmosphere.
Exceptionally high optical quality fused silica can be obtained by this method. The process takes advantage of the highly reactive nature of the porous, high surface area, soot preform. The tendency to form layered striae is essentially eliminated. The process has been employed extensively in making glass articles of small bulk, but where high optical quality is demanded, e.g., optical waveguides.
The present invention is designed to produce fused silica-containing articles of large area, i.e., large cross section or diameter, which exhibit very high optical qualities. In addition, the invention provides means for forming laminated articles in large areal bulk wherein at least one lamina consists of fused silica-containing material of very high optical quality.